HOW TO GENERATE A BUSINESS PARTNER AUTOMATICALLY

Hi in my other sap i see someone created a trigger to auto generate a business partner number. eventhough i copied the trigger to my new db i dont see the option to generate the following number. in my old database once i selected to create a customer on the code field it showes me like a search/magnifying glass and once i click on it it generates the next customer number.

does someone know how to do this.

thank you.